Several colorimetric systems have been examined in order to quantitatively deal with colors that are perceived by the human sense. The present specification employs the (L*a*b*) colorimetric system which has recently been evaluated as most suitable. The symbols a* and b* mean chromaticness indices defined in the CIE 1976 (L*, a*, b*) space (see pages 137-143, "Handbook Science of Color, New Edition", edited by the Japanese Society of Science of Color).
A color which looks black to the human eye is generally in the region defined by 0.ltoreq..sqroot.a*.sup.2 +b*.sup.2 .ltoreq.2. It is said that as this value approaches zero, the color becomes in ideal black color.
For the preparation of black color recording liquids for ink-jet recording, there have previously been known a method comprising mixing yellow, magenta and cyan as described, for example, in the published specifications of Japanese Patent Application (OPI) Nos. 5771/1982, 76071/1982, 164170/1982 and 101171/1982 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), a method comprising mixing greenish black with red or violet as described, for example, in the specifications of Japanese Patent Application (OPI) Nos. 183764/1983, 183765/1983, 183766/1983, 183767/1983, and 201863/1983, and a method comprising mixing bluish black with greenish black as described, for example, in the specifications of Japanese Patent Application (OPI) Nos. 93766/1984, 93767/1984, and 93768/1984.
However, recording liquids prepared by mixing yellow, magenta and cyan, or by mixing greenish black with red or violet have the disadvantage that when they are used in recording on paper or when recorded images get wet with water or alcohol, color separation occurs in the surrounding parts of recorded dots owing to slight differences of the individual colors in their affinity for paper and their solubility in solvents, and the quality of printed characters is markedly degraded.
The method comprising mixing bluish black with greenish black has the disadvantage that a region having a .sqroot.a*.sup.2 +b*.sup.2 value of 0 to 2 is very difficult to obtain, and the color of the recording liquid varies greatly depending upon the type of paper.